1. Field of the Invention
The present invention relates to an image reading apparatus for reading a document placed on a document positioning glass plate with a reading sensor unit mounted on a carriage.
2. Description of the Related Art
An image reading apparatus, such as a facsimile machine, reads a document placed on a document positioning glass plate with a reading sensor unit mounted on a carriage. A conventional image reading apparatus has a configuration in which a carriage including a reading sensor unit is movably guided and supported along a guide shaft formed with a metal shaft and moved by a motor provided in the apparatus. In this case, the apparatus generally has a drive transmission mechanism for the carriage, which transmits a driving force of the motor to the carriage via a gear, a timing belt, and the like. In recent years, a type for making the carriage to self-run along a rack member is used instead of the guide shaft. This type of image reading apparatus needs to have an urging member which is located opposite rack teeth of the rack member and regulates a horizontal position of the carriage to prevent play or backlash of the carriage caused by a reaction force acting on a meshing portion between the rack teeth and a pinion gear.
FIG. 9 is a plan view when a document cover of a conventional image reading apparatus is opened. FIG. 10 is a transverse cross sectional view of a conventional rack member and a sliding portion of a slider. FIG. 11 is a partial plan view of a drive mechanism for a carriage illustrated in FIG. 9. FIG. 12 is a transverse cross sectional view of a sliding portion having an urging member for reducing backlash between the rack member and the slider in FIG. 10. The image reading apparatus includes an apparatus body 101 for housing and holding various types of functional parts, a document positioning glass plate 102 for placing a document, a reading sensor 103 having a contact image sensor for reading an image of a document, and a carriage 104 including the image sensor 103 thereon and movable.
The image reading apparatus further includes a slider 105 fixed on the lower side of the carriage 104, a rack member 106 provided along the Y-direction of the apparatus body 101, and a carriage driving motor 107 mounted on the carriage 104. The carriage 104 moves while causing the slider 105 to contact and slide on a guide rail 121 provided at the rack member 106. The rack member 106 has rack teeth 122 formed over the approximately whole length thereof. The carriage 104 has a pinion gear 108, which is meshed with the rack teeth 122 and rotatably supported. The carriage 104 is a self-run type. More specifically, the driving force from the motor 107 is transmitted to the pinion gear 108 via a motor gear 124, an idler gear 125, and a worm gear 126. Further, the carriage 104 moves in the Y-direction along the rack member 106 by moving and rolling the pinion gear 108 on the rack teeth 122. The carriage 104 moves back and forth along the rack member 106 in response to the normal/reverse rotation of the motor 107.
The rack member 106 has a guide rail 121, on which the slider 105 slides when the carriage 104 moves. The guide rail 121 includes a vertical directional contacting portion 106a and a horizontal directional contacting portion 106b, and the slider 105 can slide on these two portions. The vertical directional contacting portion 106a is formed on an upper face of the guide rail 106, and the horizontal directional contacting portion 106b is formed on a back face of the rack teeth 122 of the guide rail 106.
However, in the aforementioned image reading apparatus, when the pinion gear 108 rotates in the R direction in FIG. 11, the slider 105 receives a force for rotating a front end F in the arrow T direction via the carriage 104 by a reaction force at the time of meshing the pinion gear 108 with the rack teeth 122. As the length of the moving direction of the slider 105 (the interval between portions F and E in FIG. 11) decrease in order to reduce the size of the apparatus, the force in the arrow T direction increases. Thus, as illustrated in FIG. 10, a space S is formed at the front portion (portion F) of the slider 105 by separating the horizontal directional contacting portion 105b of the slider 105 from the horizontal directional contacting portion 106b of the rack member 106 due to the rotating force. As a result, the backlash of the carriage 104 occurs in the arrow B directions illustrated in FIG. 9 around a contacting point of the vertical directional contacting portion 105a of the slider 105 and the vertical directional contacting portion 106a of the guide rail 121 in the back portion (portion E) of the slider 105. Therefore, since the backlash occurs at both right and left ends of the reading sensor 103, a high-definition reading image is difficult to acquire.
As illustrated in FIG. 12, in order to solve the problems of the configuration in FIG. 10, a configuration, in which an urging member 109 urges the slider 105 in the arrow P direction, is employed. The urging member 109 is provided in the rack member 106 via a holder 110. That is, by urging the slider 105 toward the guide rail 121 by the urging member 109 of the rack member 106, the two horizontal directional contacting portions 105b disposed in the back and front portions of the slider 105 are made contact with the horizontal directional contacting portions 106b of the rack member 106 so as to remove the space S. However, the configuration using the urging force of the urging member 109 for reducing backlash is easily affected by external factors, such as frictional variation or driving load variation, at the time of moving the carriage, and thus it is difficult to keep a stable running without backlash. Further, since the urging member 109 urges the slider 105, the driving load of the carriage 104 increases. Recently, a reading rate has remarkably improved so that increasing of the driving load is undesirable to improve the reading rate. Furthermore, since this configuration requires the urging member 109 and the holder 110, it has become one of the factors of cost increase.